1. Field of the Invention
The present invention relates to a polarization independent wavelength converter and a polarization independent wavelength conversion method.
2. Description of the Related Art
In optical transmission, the transmission capacity increases at a rate of 1.4 times/year, and it has reached over 100 Gbits/s at some nodes. With the increase in the transmission capacity, power consumption due to the optical transmission increases, so that reduction in the power consumption is becoming a problem to be solved. There is a known technology to deal with an optical signal without converting it to an electrical signal as a method of reducing power consumption. This technology splits or combines signal lights of different wavelengths in a Wavelength Division Multiplexing (WDM) signal light launched into a certain node with a wavelength selection switch and transmits the split or combined signal lights to different nodes. When the wavelengths of a plurality of signal lights transmitted to the different nodes are equal, it is necessary to convert the plurality of signal lights into different wavelengths to combine the plurality of signal lights.
As a method of optically converting a certain wavelength of a signal light into a different wavelength of a signal light, there is a method of using optical non-linearity of an optical fiber. In order to perform the wavelength conversion by Four Wave Mixing (FWM) which is one of the non-linear optical effects, an input signal light as a subject of the wavelength conversion, and a pump light having a wavelength different from that of the input signal light are input to the same optical fiber. Then, a signal light having a wavelength which is converted (hereinafter, referred to as a wavelength-converted light) is generated in the optical fiber, so that the wavelength-converted signal light as well as the input signal light and the pump light are output from the optical fiber.
In FWM, when the optical frequency of the input signal light is assumed to be νs [Hz], the optical frequency of the pump light is assumed to be νp [Hz], and the optical frequency of the wavelength-converted light is assumed to be νc [Hz], the relation “νp=(νs+νc)/2” is established. When the speed of light is assumed to be C [nm/s], the relation “λ=C/ν” is established between the optical frequency ν [Hz (=1/s)] and the wavelength λ [nm]. Therefore, when the wavelength of the input signal light is assumed to be λs [nm], the wavelength of the pump light is assumed to be λp [nm], and the wavelength of the wavelength-converted light is assumed to be λc [nm], the relation “(1/λp)=[(1/λs)+(1/λc)]/2” is established. Therefore, when the wavelength of the input signal light and the desired wavelength of the wavelength-converted light are given, the wavelength of the pump light is determined. In the following document, a signal light is successfully converted into an arbitrary wavelength in the wavelength range of from 1530 nm to 1610 nm by FWM in a High Non-Linear Fiber (HNLF) (M. Takahashi et al., “Full C-L Band Tunable Wavelength Conversion by Zero Dispersion and Zero Dispersion Slope HNLF,” ECOC2009, P1.08, 2009. hereinafter to be referred to as Nonpatent Reference 1).
In order to improve the efficiency of generation of the wavelength-converted light by FWM (or wavelength-conversion efficiency), a polarization of the input signal light and a polarization of the pump light are preferably adjusted to match each other. In Nonpatent Reference 1, the polarization of each signal light is adjusted by a polarization controller so that the polarizations of two signal lights may match each other.
However, it is preferable that the wavelength conversion is performed without using the polarization controller to use the wavelength converter in an optical communication system. It is because the polarization of the input signal light may have various polarization states. Adding an automatic polarization regulator that regulates the polarization controller in accordance with the polarization state of the input signal light to the wavelength converter is considered to be practically disadvantageous from the viewpoint of manufacturing cost and packaging area or volume. Moreover, the reliability of operation improves by reducing the number of devices which have to be controlled during the operation of the wavelength converter.
Japanese Patent No. 3476806 (hereinafter to be referred to as Patent Reference 1) discloses a method in which a polarization maintaining optical fiber is used as an optical fiber to generate FMW, as a method of exhibiting a constant wavelength-conversion efficiency regardless of the polarization state of the input signal light even without using the polarization controller.